In recent years, attention has been paid to a technique of comprising a thin film transistor (TFT) by using a semiconductor thin film (thickness around several to several hundreds nm) formed on a substrate having an insulating surface. The thin film transistor is widely applied to an electronic devices such as IC or an electro-optical device, and has been rapidly developed especially as a switching element of an image display device.
Various applications using such an image display device are expected, and in particular utilization for a portable device draws attention. At present, although a lot of glass substrates or quartz substrates are used, these are easily broken and heavy, which are defects. In addition, a glass substrate or a quartz substrate is difficult to enlarge and unsuitable in a mass production. Therefore, forming TFT element on a substrate with flexibility, representatively, on a flexible plastic film is attempted.
However, it is inevitable to lower the highest temperature of a process because the heat resistance of a plastic film is low, and as a result, it is impossible to form TFT with electric properties as favorable as the time forming on a glass substrate, which is the present state. Therefore, a high performance liquid crystal display device or light-emitting element using a plastic film is not realized.
In addition, a peeling method for peeling a layer to be peeled existing over a substrate through an isolation layer from the substrate, has been already suggested. For example, the patent document 1 and the patent document 2 describe a technique of separating a substrate by providing an isolation layer of amorphous silicon (or polysilicon) and irradiating a laser light through the substrate to release hydrogen contained in the amorphous silicon to form air gap. Moreover, the patent document 3 gives a description of sticking a layer to be peeled (in the publication, called a layer to be transferred) to a plastic film with the use of this technique to complete a liquid crystal display device.
However, in the aforementioned method, it is essential to use a highly transparent substrate, and irradiation of a relatively large laser light is required to pass through the substrate and then to release hydrogen contained in the amorphous silicon, which cause some damage to a layer to be peeled. Further, in the aforementioned method, in a case of forming an element on an isolation layer, hydrogen contained in the isolation layer is diffused and decreased if a heat treatment at a high temperature is carried out in the process of manufacturing the element. As a result, there is a possibility of insufficient peeling even if a laser light is irradiated at the isolation layer. Therefore, there is a problem that a process after forming the isolation layer is limited to keep the amount of hydrogen contained in the isolation layer. Although the aforementioned publication gives a description of forming a light-shielding layer or a reflection layer in order to prevent the layer to be peeled from being damaged, in that case, it becomes difficult to fabricate a transmission type liquid crystal display device. Moreover, in the aforementioned method, it is difficult to peel a layer to be peeled with a large surface.
Patent Document 1: Japanese Patent Application Laid-Open No. H10-125929
Patent Document 2: Japanese Patent Application Laid-Open No. H10-125931
Patent Document 3: Japanese Patent Application Laid-Open No. H10-125930